There are a wide range of situations where substrates come into contact with fouling agents that give rise to deposition onto the substrate over time. For example, fouling is common in marine and aquatic environments, on substrates such as domestic appliances, glass or other surfaces. Heat exchangers and other machinery that come into contact with water (particularly hard water) will be subject to fouling or scaling over time and many components of food and beverage processing equipment and other industrial machinery or appliances will often experience unwanted plaque build-up or fouling. Depending upon the context, fouling of substrates can be unsightly, can give rise to hygiene or health and safety issues, can necessitate costly down time of equipment and maintenance/cleaning costs as well as reducing the efficiency of equipment operation. There is therefore a pressing need, and significant commercial motivation, to develop technologies capable of preventing or reducing fouling of substrates on exposure to fouling agents such as food and beverages, industrial chemicals, water, milk and other dairy products, marine or aquatic environments, sewage and the like.
The dairy industry is one that is particularly affected by the fouling of equipment, requiring frequent and expensive cleaning steps to restore equipment performance following fouling. Not only are the cost of cleaning and the down time of equipment significant problems, but the necessary cleaning steps require the use of water, energy and chemical cleaning agents such as strong acids and/or alkali that are not environmentally friendly.
Milk fouling in the dairy industry is particularly severe due to the thermal instability of the milk system (Changani and Belmar-Beiny 1997). The literature suggests that protein and minerals may be all involved in the occurrence of milk fouling, which starts with surface adsorption and involves different mechanisms under different conditions (temperature and flow pattern) (Burton 1968; Delsing and Hiddink 1983). Heat induced reactions then take place to build up fouling layers to eventually form milk stones (de Jong and Bouman 1992; Delplace, Leuliet et al. 1997; Chen and Bala 1998; Chen and Chen 2001; Bansal and Chen 2006).
Milk deposits can be characterised with respect to processing temperature as Type A and Type B deposits. Type A deposits are found at temperatures below 110° C., and consists of 50-60 wt % proteins and 30-35 wt % minerals, which are much higher proportions than those found in raw milk. The Type A deposit is creamy and white and is known as protein fouling. However, if it is overcooked it can become brown in colour and very much harder. Type B deposits are found at heating temperatures above 110° C., and consist of 15-20 wt % protein and up to 70 wt % minerals (Lalande, Tissier et al. 1984). The major mineral compound is understood to be calcium phosphate. This type of deposit is harder than the Type A deposits, is grey in colour and is known as mineral fouling (Burton 1968).
The unwanted deposition on the surfaces of heat exchanger apparatus (in both the dairy industry and in other contexts) represents an additional thermal resistance to heat transfer, which reduces the thermal-hydraulic performance for the heat transfer equipment.
One approach that has been considered in attempts to reduce surface fouling, for example in the dairy industry, is to change the characteristics of the heat exchanger surface in the hope of altering the interaction with the fouling agent that leads to adsorption of the first deposition layer. The theory is that as the base layer structure is changed, the subsequent fouling reactions would also be altered and hopefully inhibited (Liu, Chan et al. 2010). In the past, anti-fouling coating technologies such as Ni-P-PTFE coatings, Xylan®, Silica, SiOx, Exvalibur® and Diamond-like Carbon (DLC) coatings have been tested in order to reduce the milk fouling during thermal treatment. While such coatings have changed the fouling behaviour of heat exchangers coated by these means, the results have not been commercially acceptable (Beuf, M., G. Rizzo, et al. (2004). For example, the reduction in fouling has either not been significant or the coatings have resulted in other problems such as de-lamination or shedding into the product stream, degradation of the substrate or product contamination.
Water scaling is problematic in many industries, particularly where hard water is involved. Scale on a heat exchanger surface generally produces a higher resistance to heat transfer. In cooling water applications, hard water calcium and magnesium form combinations that come out of solution easily and form unwanted deposits (Sultan Khan, Zubair et al. 1996). With alteration of surface characteristics, it may also be possible to minimise the effects of water scaling.
In view of this background it is desired to develop a means of preventing or reducing the fouling experienced on a substrate when it comes into contact with a fouling agent. For example it would be useful to develop a means of preventing or reducing the fouling that takes place on a range of different substrates and which is caused by exposure to a variety of different fouling agents.
Other aspects of the present invention will become apparent form the following detailed description.